Blood Clots During Menstrual Periods

Why period blood forms clots

During your period, the lining of the uterus (endometrium) sheds along with blood. Your body releases natural anticoagulants that keep menstrual blood liquid as it leaves. When blood flows quickly, or pools in the uterus or vagina before it is passed, those anticoagulants cannot keep up and clots form. That is why clots appear most on heavy days, and often when you stand up after lying down overnight.

Clots are usually dark red or almost black. They may contain pieces of endometrium, so a clot is not always pure blood.

Normal vs abnormal clots

Causes of large or frequent clots

Uterine causes

• Uterine fibroids: non-cancerous muscle growths that enlarge the uterus and its lining. Fibroids that bulge into the cavity are a common cause of heavy, clotty periods.
• Adenomyosis: lining tissue growing into the uterine wall, causing heavy, painful periods and a bulky, tender uterus.
• Endometrial polyps: overgrowths of the lining that can cause heavy bleeding and bleeding between periods.
• Endometrial hyperplasia or cancer: thickened lining, mainly over 45, often with irregular bleeding.

Hormonal causes

• Perimenopause: cycles without ovulation let the lining build up, then shed heavily.
• Polycystic ovary syndrome: infrequent ovulation causes infrequent, sometimes very heavy periods.
• Thyroid disorders, especially an underactive thyroid.

Bleeding disorders and medicines

• Von Willebrand disease: the most common inherited bleeding disorder, often causing heavy periods with clots from the first period onward, plus nosebleeds and easy bruising.
• Blood thinners: warfarin and newer anticoagulants commonly make periods heavier.
• Copper IUD: can increase flow and clots, especially in the first year.

Pregnancy loss

A miscarriage can look like a heavy, painful period with large clots or gray-pink tissue, sometimes before you knew you were pregnant. Take a pregnancy test if your period was late. An ectopic pregnancy is less likely to cause large clots but can cause bleeding with one-sided pain.

Symptoms that come with clots

• Severe cramps: clots stretch the cervix as they pass, which hurts. Painful periods with heavy clots suggest fibroids or adenomyosis.
• Tiredness, breathlessness, cold hands, hair thinning and pale skin: iron deficiency from blood loss. See iron deficiency anemia.
• Pelvic pressure or frequent urination: large fibroids.
• Bleeding between periods or after sex: polyps or cervical problems.

How doctors find the cause

A doctor will ask about clot size, how often you change protection, period length, pain, contraception, pregnancy possibility, and family history of bleeding. A period diary for 2 or 3 cycles is very useful. Tests usually include:

• a complete blood count and ferritin (iron stores)
• a pregnancy test
• thyroid function, and clotting tests or von Willebrand studies if heavy periods began at your first period
• a pelvic exam and transvaginal ultrasound
• sometimes hysteroscopy or an endometrial biopsy, especially over 45

Treatment

• Tranexamic acid: a non-hormonal tablet taken on heavy days that helps blood clot at the source, reducing flow by up to about half.
• NSAIDs: ibuprofen or naproxen during the period reduce both flow and cramps. Follow the label dose and avoid them if you have stomach ulcers or kidney disease.
• Levonorgestrel hormonal IUD: the most effective long-term treatment for heavy periods.
• Combined pill or progestins: lighten and regulate periods.
• Iron supplements for low iron or anemia.
• Procedures: removal of polyps or fibroids, endometrial ablation, or hysterectomy when other treatments fail and no future pregnancy is wanted.

Frequently asked questions

Are blood clots during a period normal?

Yes, small clots are normal, especially on the heaviest days of your period. They form when blood flows faster than your body's natural anticoagulants can work. Frequent clots larger than a quarter, or clots with very heavy flow, deserve a doctor's check.

What size of period clot is too big?

Clots larger than a quarter, about 2.5 cm or 1 inch across, are considered large. Passing them now and then on a heavy day is not unusual, but if it happens most periods or with flooding, it is a sign of heavy menstrual bleeding.

Why am I suddenly passing large clots in my period?

New large clots can come from perimenopause, fibroids, polyps, a new copper IUD, blood thinners or thyroid changes. If your period was late, a miscarriage is possible. See a doctor if the change persists over 2 or 3 cycles, or immediately if bleeding is very heavy.

Can period clots be a sign of miscarriage?

They can. A miscarriage can look like a heavy period with large clots, tissue and cramps, often after a late period. If you could be pregnant, take a test and contact a doctor. Seek emergency care for very heavy bleeding or severe pain.

Do period clots mean I have anemia?

Not by themselves, but frequent large clots usually mean heavy blood loss, which commonly leads to iron deficiency. Tiredness, breathlessness on stairs, headaches and pale skin suggest anemia. A simple blood test checks hemoglobin and iron stores.